fix(api): hide sensitive fields by default

Add model-level hidden fields for secrets, tokens, keys, notification
credentials, deployment logs, and environment values. Allow explicit
read:sensitive API access to reveal gated private keys and deployment logs,
and cover the behavior with feature and unit tests.

app/Models/SlackNotificationSettings.php

@@ -33,6 +33,10 @@ class SlackNotificationSettings extends Model
         'traefik_outdated_slack_notifications',
     ];
 
+    protected $hidden = [
+        'slack_webhook_url',
+    ];
+
     protected $casts = [
         'slack_enabled' => 'boolean',
         'slack_webhook_url' => 'encrypted',